Loop powered level measurement systems operate on a 4-20 mA current loop, hence the name loop powered. The circuitry for the level measurement system, i.e. the load, is typically designed to operate at less than 4 mA. The current loop provides a terminal voltage in the range 12-30V, but is nominally 24V.
To take a measurement, power is applied to the transducer and the reflected pulses are detected and the distance to the reflective surface is calculated or measured. If more than 4 mA is needed to make a measurement, then energy taken from the current loop is stored until there is enough to make the measurement. In addition, to make rapid measurements, more current from the loop is also needed. As the current in the loop increases, the speed of measurement also increases. Since the power available from the current loop is less than the power required to continuously operate the level measurement device, the level measurement device is operated intermittently. In a typical level measurement system, measurements are taken once every second up to once every five seconds.
In a typical level measurement application, the measurement instrument is mounted inside the storage tank or vessel, and a remote receiver, for example an industrial controller or plant control computer, is coupled to the other end of the current loop. If the level measurement system is being used in explosive or hazardous environments, for example in a chemical plant or refinery, protection circuitry is provided to avoid a potential explosion, for example, if there is sparking in the circuitry in the measurement instrument. The protection circuitry also acts to prevent damage from high voltage transients or when two different circuits show different ground potentials.
One form of known protection circuitry utilizes Zener diodes. The Zener-based protection circuitry restricts the maximum voltage that could be found in the circuit. A resistor is typically added to further reduce potential energy build-up which may result in sparking. However, Zener diodes exhibit a leakage current, and the leakage current will vary depending on the applied voltage. One approach is to include a regulator to control the voltage applied across the Zener diodes to minimize leakage currents. While this approach has proven adequate to reduce and even minimize the leakage currents, component variations in the circuitry and temperature changes still result in variations in the leakage currents. Since a loop powered level measurement device requires precisely controlled currents, it will be appreciated such variations will affect the precision of the device.
Accordingly, there remains a need for improved method and apparatus for stabilizing leakage current variations so that the loop current is accurate over temperature and input voltage.